Furfurals are important intermediates for synthesis of value-added chemicals. Furfural and related compounds, such as hydroxy methyl furfural (HMF), are useful precursors and starting materials for industrial chemicals for use as pharmaceuticals, herbicides, stabilizers, and also for making polymers.
While furfural is commercially produced today, HMF is not produced on an industrial scale. One current furfural manufacturing process utilizes biomass such as corn cobs as a raw material feedstock for obtaining xylose or hemicellulose. The hemicellulose is hydrolyzed under acidic conditions to its monomer sugars, such as glucose, fructose, xylose, mannose, galactose, rhamnose, and arabinose. Xylose, which is a pentose (i.e., a “C5 sugar”) is the sugar present in the largest amount. In a similar aqueous acidic environment, the C5 sugars are subsequently dehydrated and cyclized to furfural.
A major difficulty with known methods for dehydration of sugars is the formation of undesirable resinous material that not only leads to yield loss but also leads to fouling of exposed reaction vessel surfaces and negatively impacts heat transfer characteristics. A review by R. Karinen et al. (ChemSusChem 4 (2011), pp. 1002-1016) includes several commonly used methods of producing furfural generally as described above. All of those methods involve use of a soluble inorganic acid catalyst, such as sulfuric, phosphoric, or hydrochloric acid. These acids are difficult to separate from the final product. Low yields can result from formation of undesirable acid byproducts and resinous material called humins. Mass transfer limitations may also impose restrictions on furfural yield, especially in the use of solid biomass as feedstock. Further, the use of mineral acids requires special materials of construction due to associated corrosion issues, thereby incurring increased capital costs. Environmental emission issues are also of concern. During the commercial production of furfural, yields of only about 50-55% are typically achieved.
WO 2011/126654 discloses a biomass pretreatment method in which the biomass material is subjected to a biphasic mixture of water and supercritical CO2 at a temperature in the range of 150° C. to 250° C. under high pressure (74 to 300 bar, 7.4 to 30 MPa) for a time of from 10 seconds to 100 minutes. In particular embodiments, the process is performed as a two-stage temperature process, wherein an initial, short high-temperature stage is conducted at a temperature of at least 200° C. for up to 20 minutes, and a subsequent, longer low-temperature stage is conducted at a temperature of at least 140° C. and up to 190° C. for 10 to 120 minutes. The process disclosed is designed to maximize sugar yields while minimizing the conversion of these sugars to furfurals. Furfurals are considered to be an unwanted byproduct.
There remains a need for processes for producing furfural and related compounds from sugars at high yield, while minimizing the formation of other byproducts such as acids and undesirable humins.